1. Field of the Invention
The present invention relates to drop generators used to generate uniform droplets used for printing on a recording surface.
2. Prior Art
In an ink jet printing system, a pressurized volume of print fluid such as ink is supplied into the ink receiving cavity of a drop generator. The ink is extruded as one or more capillary streams through one or more orifices coupled to the ink receiving cavity. A crystal which is disposed relative to the ink cavity is excited and creates a perturbation so that the streams are broken up into a plurality of droplets. The droplets are then controlled for writing on a recording surface.
It is desirable that the droplets produced from the streams passing through each of the nozzles have substantially the same break-off point, be substantially uniform in size, have substantially uniform spacing between the droplets, and be satellite-free. This ensures that the quality of the print from each of the nozzles will be substantially the same.
To obtain this uniformity between the drolets of the various streams, it is necessary that the perturbations applied to each of the ink streams of the nozzles be substantially uniform and that the nozzles be of uniform quality. Furthermore, for the production of the droplets to be satellite-free, it is necessary that the perturbations be sufficiently large. It also is necessary for the perturbations to not only be substantially uniform but to be reproducible throughout the time that the droplets are being produced.
To meet these basic requirements, it is necessary that the transducer or driver, which produces the vibrations for causing the perturbations in the ink streams, be capable of operation so that the amplitude of each of the pressure waves produced in the ink cavity by the driver is substantially the same at the entrance to each of the ink jet nozzles. This will produce uniform perturbations in the ink jet streams flowing through the nozzles. It also is necessary for the amplitude of the pressure waves to be sufficiently high to produce satellite-free droplets.
The prior art abounds with drop generators whose designs and/or configurations strive to achieve the aforementioned qualities. U.S. Pat. No. 4,153,901 describes a multinozzle drop generator wherein a hemicylindrical or half cylinder crystal is used to create the disturbance. The drop generator consists of a carrier base or back plate in which an ink cavity and ink supply lines are fitted. The cavity is filled with a layer of resonance attenuating compound such as epoxy and Teflon. The teflon/epoxy layer is needed to attenuate unwanted resonances and reflections which affect the efficiency of the drop generator with frequency changes. The hemicylindrical crystal is mounted in the cavity with its concave surface facing upwardly. A gasket is fitted over the crystal and seals the cavity forming an ink chamber. A nozzle plate having a plurality of nozzles is then fitted over the gasket. A front plate with an elongated slot is fitted over the nozzle plate. The slot is aligned with the orifices. The components are held in position against the back plate by support screws.
The major problem with the hemicylindrical drop generator is that the drop generator is nonextendable. The term nonextendable means that neither the length of the nozzle array (that is the number of nozzles needed for printing) nor the drop frequency (that is the frequency used to drive the crystal) can be changed without undue degradation in the performance of the drop generator. Degradation includes nonuniform break-off of droplets, satellite problems, etc. By using the layer of resonance attenuating compound, the prior art tends to improve the probability for limited frequency change. However, the range of the frequency change is very limited. Moreover, the prior art does not address the problem of increasing the number of orifices in the nozzle plate.
The use of a resonating attenuating compound in the prior art drop generators tends to increase the overall cost of the drop generator. The cost increase stems from increase in assembly time and the cost of the layer.
As is well known to those having ordinary skill in the art, in order to reproduce copies with acceptable print quality, any change in the speed of the transport used to transport paper past the drop generator requires a change in the drop frequency. Also, changes in the print resolution requires changes in the drop frequency. It is therefore obvious that the prior art which has the capability to operate at a single frequency or at most, within a range of limited frequency change, is not suitable for use in several types of ink jet printers. In other words, the prior art drop generators tend to impose undue limitations on the overall design and operation of the entire ink jet printer.
Another type of prior art drop generator for use in an ink jet printer is described in U.S. Pat. No. 3,958,249. Pressurized print fluid such as ink is supplied to a tube having a nozzle plate with an orifice communicating with the interior of the tube. A cylindrical radially contracting and expanding transducer surrounds the tube and the nozzle plate. When a signal is applied to the transducer, the perturbations change the cross-sectional area of the tube and/or orifice, and as a result, the stream emitted from the orifice is broken up into droplets.
The main problem with this type of drop generator is that the tube and/or nozzle plate must be deformed. As such, a relatively large amount of power is required. Also, it would appear as if the invention has limited use with a single nozzle head. Invariably with a multinozzle head, it would be impractical to encase the head with a cylindrical transducer. Moreover, the power requirement for such a configuration would be prohibitively high.
Yet another type of prior art drop generator is described in U.S. Patent 3,334,351. In the patent, two separate transducers arranged at different angles, input dual motion to a single nozzle. The arrangement is manifestly inefficient. Moreover, when applied to a multinozzle head, the arrangement would result in a complex motion, making attainment of uniform drop break-off for all streams extremely difficult.